Radio receiving system



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G. J. LEHMANN Filed Oct. 4, 1943 Dec. 24, 1946.

' 'RADIb RECEIVING SYSTEM INVENTOR.

ATTORN mskbluk NQ l atented Dec. 24, 1946 UNITE STATES PATENT OFFICE2,412,994 RADIO RECEIVING SYSTEM Application October 4, 1943, Serial No.504,822 In France August 29, 1941 1 Claim.

This invention relates to radio receiving systems and more particularlyto a system for receiving waves of time modulated pulses.

It is known that for the reception of electrical energy which is pulsemodulated, wherein the pulses occur at substantially equal intervalsapart, a receiver which is synchronously blocked and unblockedconstitutes an efiective protection against high level interference.This synchronous blocking and unblocking is accomplished by use ofsynchronous resonators which provide regularly timed unblocking pulsesfor control of the receiving circuit which is normally blocked. Thisincreases the signal-to-noise ratio because the receiving circuit isonly unblocked for short intervals corresponding to the approximatetiming of pulses, the reception of which is desired. In other Words, thereceiving circuit is blocked for substantially the duration of theintervals between succeeding pulses thereby eliminating interferenceoccurring during those intervals.

While the prior art systems for controlling the reception of receiversis efiicient for waves of time modulated pulses having substantiallyequal intervals between successive pulses, such syste s are not soefficient for receiving double or push-pull time modulated pulses. Thismethod of time modulation pairs off the pulses in that the timemodulation of the pulses of each pair consists generally in thedisplacement of the two pulses of each pair in opposite directions. Thatis to say, the pulses of each pair are displaced either toward or awayfrom each other in pushpull manner in accordance with the instantaneousamplitude of the signal energy. When the aforesaid prior art method ofblocking and un blocking a receiver circuit is applied to a receiver forreception of push-pull modulated pulses, the unblocking pulse would bedisplaced at equal intervals apart while the pulses of each pair wouldbe displaced by an interval either greater or less than the intervalbetween the trailing pulse of one pair and the leading pulse of the nextsucceeding pair. This means that a much wider unblocking pulse isrequired increased by an amount equal to the difference between the twointervals resulting in a lower signal-to-noise ratio.

It is an object of this invention, therefore, to provide a receivingsystem with means for improving greatly the signal-to-noise ratio forreception of waves of double or push-pull time modulated pulses.

The above and other objects of the invention will become more clear uponconsideration of the following detailed description to be read inconnection with the accompanying drawing, the sole figure of which showsin block diagram, a receiving system according to the principles of thisinvention, together with a pair of curves A and B used in explaining theinvention.

Referring to the drawing, curve A represents a train of push-pullmodulated pulses l to I!) which are characteristically grouped in pairssuch as pulses l, 2, 3, 4, 5, 6, etc., according to the pushpull type ofmodulation disclosed in the copending application of Bac-Bonhomme andmyself entitled, Electrical signalling system employing pulsemodulation, Serial No. 491,708, filed June 21, 1943. .The pulses of eachpair are displaced in opposite directions according to the instantaneousamplitude of the modulating signal energy. The degree of displacement issmall compared to the duration of the pulse. The interval betweensucceeding pulses may be 20 microseconds more or less depending upon thefoundation wave and the normal bias, if any, imposed upon the modulatorat the transmitter. Should the modulator at the transmitter be normallybiased, the pairs of pulses will be spaced apart with the time intervalbetween the trailing pulse of one pair and the leading pulse of the nextpair greater than the interval between the two pulses of each pair.Thisis shown in curve A wherein interval a between the pair of pulses land 2 is smaller than the interval 1) between pulses 2 and 3. Thisparticular pulse relationship as well as the dimension of the intervalspacings are given by way of example only since both may obviously bewidely varied without departing from the invention. For example, whereno bias is used, the intervals between succeeding pulses will be equalwhen the pulses are unmodulated and unequal when some degree ofmodulation is applied thereto.

The receiver system shown in the drawing includes the usual form ofdetector 20 which receives pulse modulated carrier wave energy fromantenna 22 and translates the energy into current pulses. The output ofthe detector 20 is applied to a threshold limiter 24 whereby low randomnoise fluctuations are eliminated. The more pronounced randominterference pulses such as 25 and 26, curve A, will be passed by thethreshold limiter 24 along with the signal pulses. These randominterference pulses, however, are usually entirely eliminated by thesystem since any two such interference pulses are not likely to bespaced apart according to the 3 retardation characteristic of theunblocking system.

The output of the threshold limiter 24 applies the train of pulsesrepresented by curve A to parallel circuits 30 and 31 each of whichterminates at a known form of demodulator 35.. The circuit 3| includesin series connection an amplifier 32, a delay device 33 and a limitclipping amplifier 34. The received pulses such as those illustrated incurve A are applied to the amplifier 32 where they are amplified asindicated by pulses 1a and 8a. The amplified pulses are fed to the delaydevice 33 of known character whereby they are retarded an amountpreferably equal to the interval which is approximately equal to the sumof the intervals (1 and b (curve A). The pulses thus retarded are alsoincreased slightly in duration because of the attenuatingcharacteristics of the delay device. Assuming that the curves A and Bhave the same time base, the pulse energy of pulses 3 and 4 passing overcircuit 3| will appear retarded by an interval 0 as indicated by thepositions of pulses 3a and 4a of curve B. These retarded pulses arelimit clipped by amplifier 34 thereby producing rectangular pulses suchas indicated at Ia and 2a corresponding in time with the pulses 3 and 4transmitted over circuit 30, also that the pulse energy corresponding topulses 5 and. 6 is at the same moment present within the delay device33.

It will be understood that a very large number of pulses are used todefine even a small portion of the modulating signal energy so that thedifference in displacement of succeeding pulses is very small.Consequently, alternate pulses such as I and 3 or 2 and 4 are generallydisplaced nearly the same amount in the same direction from theirunmodulated positions. By retarding energy of pulse l by an amount 0 (oran amount slightly under 40 microseconds for the example given) which issubstantially equal to the diiference in time between pulses I and 3, anunblocking pulse la is produced which extends in duration sufiicient tocover the position of pulse 3. By regarding pulses ia, 211, 3a etc., asthe windows for pulses 3, 4, '5, etc., it will be readily understoodthat these window pulses ocour in coincidence with the correspondingsignal pulses so that the demodulator will pass the energy of the signalpulses-and block interfer-' ence pulses such as pulses 25 and 26. Y

It will thus be understood that the unblocking pulse produced for anysignal pulse by the preceding alternate pulse will be time modulated inthe same direction and for substantially the same amount that the signalpulse is modulated. Thus, the window or unblocking pulse used accordingto my invention is as narrow in width as possible thereby resulting in avery high signal-to-noise ratio.

In sharp contrast to the efficiency of my unblocking invention for thereception of push-pull time modulated pulses is the synchronizingresonators used for producing unblocking pulses at identical intervalsapart a heretofore proposed.

In order to use such resonator principle in reception of push-pull timemodulated pulses, the unblocking pulses must be increased in duration anamount equal to the maximum possible difference in the succeedingintervals between the pulses.

The demodulator 35 is of known form whereby the time modulated pulsesare translated into pulses varying in energy according to the timemodulation of the signal pulses. The input stage of the demodulator isnormally biased by a biasing potential source 36 to block demodulatingoperation, the unblocking pulse energy Ia, 2a, 30, etc., operating toovercome suificiently the blocking bias for proper response to signalpulses for the duration of each unblocking pulse.

While I have shown the principles of my invention in connection withspecific apparatus, it is to be understood that the illustrations aregiven by way of example only and not as limiting the scope of theinvention as set forth in the objects and the appended claim.

What is claimed is:

In a system for receiving time modulated pulses of the push-pullmodulated character wherein the pulses are paired ofi with the pulses ofeach pair generally time displaced in opposite direction according tothe instantaneous amplitude of the signal energy, a demodulator, asource of signal pulses, biasing means to render said demodulatornormally non-responsive to signal pulses, a first electrical pathconnecting said source to said demodulator and a second electrical pathconnecting said source to said demodulator compris ing a delay devicearranged to extend slightly the duration of the pulses and to retard thepulses an amount substantially equal to the interval separatingalternate pulses, an amplifier preceding said delay device to increasethe amplitude of pulses before they are applied thereto, a limitclipping amplifier for limiting the amplitude of said retarded pulses soas to give them a substantially rectangular shape and means for applyingsaid retarded pulses to said demodulator to overcome said biasing meansand thereby render said demodulator responsive to signal pulses for theduration of each of said retarded pulses.

GERARD LEHMANN.

